The invention relates to an electric lamp comprising:
a lamp vessel sealed in a vacuum-tight manner and consisting of glass having an SiO.sub.2 content of at least 95% by weight:
an electric component arranged within the lamp vessel,
current supply conductors extending through the wall of the lamp vessel to the electric component,
at least one current supply conductor mainly consisting of tungsten and having a continuous coating of glass having an SiO.sub.2 content of at least 95% by weight, which coating extends from the exterior to the interior of the lamp vessel and forms with the current supply conductor a glass/metal interface, while
the surface of the coating encloses with the coated surface of the current supply conductor at the points at which they meet an angle .alpha. of at most 90.degree..
Such a lamp is known from U.S. Pat. No. 4,171,500.
In this known lamp, stringent requirements are imposed on the thickness of the coating. This thickness d must be so small that it corresponds to the formula D(d+2d).sup.-1 &gt;0.7, where D is the diameter of the current supply conductor. The thickness of the coating is therefore allowed to be only at most 21% of the diameter of the current supply conductor. Especially in case this conductor must be thin, for example 0.7 or even 0.2 mm, therefore only an extremely small thickness of the coating is admissible (at most 0.14 mm and 0.04 mm, respectively). In the preferred case mentioned in the said Patent Specification, in which D(D+2d).sup.-1 .gtoreq.0.85, i.e. d.ltoreq.0.09 D, even a thickness only 0.06 and 0.02 mm, respectively, is then admissible. This forms a serious drawback for the manufacture of the known lamp in mass production. It should be noted that the range of 0.2 to 0.7 mm is very usual for a thickness of internal current supply conductors welded to a metal foil embedded in the wall of the lamp vessel.
In the known lamp according to the aforementioned U.S. Pat. No. 4,171,500, the coating must moreover be enclosed between its ends by a thick envelope, a thick second coating, of similar glass. The necessity of this envelope forms due to the additional fusion step necessarily ensuing therefrom a further limitation for the industrial application of the known lamp.
The wall of the lamp vessel is fused in the known lamp with the said envelope, but in such a manner that the envelope has both inside and outside the lamp vessel a surface extending parallel to the surface of the current supply conductor. This results in that the current supply conductor is sealed into glass over a comparatively great length. An associated consequence is that in the lamp vessel around the current supply conductor there is a comparatively large space, which--due to its comparatively low temperature during operation of the lamp--can influence the light production of the lamp.
In substantially all types of electric lamps comprising a lamp vessel of glass hving an SiO.sub.2 content of at least 95% by weight, the current supply conductors are passed in a vacuum-tight manner through the wall of the lamp vessel in that the current supply conductors include a foil-shaped part of molybdenum, which is embedded in a pinched seal of the lamp vessel. In this construction, the foil-shaped part must be connected to the conductor extending into the interior of the lamp vessel and to the conductor extending from the pinched seal to the exterior, for which purpose welding connections must be established. Due to the ohmic resistance of the foil-shaped part, not only electric losses, but also a harmful development of heat occur in the pinched seal. The current supply conductor is moreover a slack assembly, which can be manipulated only with difficulty during the manufacture of the lamp and which makes it difficult to position accurately in the lamp vessel the part that becomes located within said lamp vessel. The accuracy of positioning could be improved if the current supply conductor with a foil-shaped part could also be held and continuously be positioned within the lamp vessel during the manufacture of a first pinched seal of the lamp vessel. During the manufacture of a second seal, a rigid current supply conductor would then have to be used. Another disadvantage of lamps having a pinched seal is that the seal is destroyed at a comparatively low gas pressure of about 80 bar. In spite of these disadvantagesm pinched seals are generally used in lamps commercially available. Excepted are only short arc discharge lamps.
In short arc discharge lamps, a construction is used, in which the current supply conductor is sealed into glass having a comparatively high expansion coefficient, which is connected via glasses having decrementally decreasing expansion coefficients to the glass of the lamp vessel, which has a very low expansion coefficient. This so-called "graded seal" obtained with the use of so-called "transition glasses" is expensive and can mostly be realized only manually. Moreover, the construction occupies a large amount of space.
GB 2,064,216-A discloses an electric lamp, in which the current supply conductors have a continuous coating of a transition glass having an expansion coefficient in the range of 11-17.times.10.sup.-7 K.sup.-1. These glasses contain besides about 81-87% by weight of SiO.sub.2 also a comparatively large quantity of B.sub.2 O.sub.3 and Al.sub.2 O.sub.3. Since these glasses have a comparatively low softening temperature, it must be avoided by forming an embossed part on the surface of the pinched seal in which the coated current supply conductors are included that the coating of comparatively low viscosity is removed during the manufacture of the pinched seal from the conductor by the quartz glass of the lamp vessel of comparatively high viscosity. Consequently, the known lamp necessarily has a profiled seal, which may be disadvantageous when mounting the lamp vessel in a lamp cap. Moreover, the comparatively low SiO.sub.2 content of the transition glass may involve the risk of giving way to attack by the gas filling of the lamp. The maximum permissible temperature of the glass, moreover, is only about 700.degree. C.
The construction having a foil-shaped part and the construction having a graded seal are used because glasses having an SiO.sub.2 content of at least 95% by weight, such as, for example, quartz glass and "Vycor", i.e. a glass containing 96% by weight of SiO.sub.2, have a linear expansion coefficient which is considerably smaller (in the range of about 4.times.10.sup.-7 K .sup.-1 to about 12.times.10.sup.-7 K.sup.-1) than that of tungsten (about 45.times.10.sup.-7 K.sup.-1). This great difference in expansion coefficient and the great difference between the softening temperature of the glasses and the operating temperature of the lamps on the one hand and room temperature on the other hand result in that tungsten cannot be included in a vacuum-tight manner in these glasses without special steps being taken.
For several decades attempts have been made to obtain special measures by which tungsten could be sealed into glasses, such as quartz glass. The result of the examinations is that commercially available lamps in such glasses still have either a pinched seal with an embedded metal foil or a graded seal.
The construction according to the aforementioned U.S. Pat. No. 4,171,500 is not used either. In spite of the mechanical strength the construction according to this Patent Specification can have the disadvantages mentioned with respect to this construction are apparently too serious. It has further been found that it is difficult to manufacture the construction described in a reproducible manner. It has been found that the reproductibility is associated with the extent to which a coating of, for example, quartz glass on the current supply conductors, which adheres to the conductors, can be obtained in a reproducible manner.
U.S. Pat. No. 3,448,320 discloses an electric incandescent lamp having a tungsten current supply conductor of at most 0.1 mm thickness, which is directly sealed into the wall of a quartz glass lamp vessel. It is emphasized that no layer of impurities must be present on the tungsten conductor. The tungsten conductor is brought into a non-oxidized state and is degased by heating at 1750.degree. to 2200.degree. C. in nitrogen or rare gas. However, the lamp described is not commercially available. The maximum thickness of the conductor is further too small for practical applications.
U.S. Pat. No. 4,086,075 discloses a method of providing a vitreous coating on metal wires. The method consists in that a metal wire together with a glass tube rightly fitting around it is heated in the high-frequency field in a protective gas, such as nitrogen. The high-frequency field can be produced by a coil connected to a current source. A non-shortcircuited coil is present in the high-frequency field, which coil is heated, like the metal wire, by the high-frequency field. They both heat the glass tube to this melting point. The coated wire is free of oxides; impurities have not been able to accumulate between the wire and the coating. By this method, according to the said Patent Specification, also vitreous coatings can be provided on wires of thoriated tungsten, which was not possible with prior methods because thorium oxide diffused to the surface of the wire and prevented a gas-tight adhesion of the glass to the wire. If a thoriated tungsten wire acting as electrode was necessary, a butt weld had to be formed thereon between the thoriated tungsten wire and a tungsten wire free of thorium oxide and the latter wire had to be provided with a glass coating.
The adhesion of a vitreous coating to a tungsten conductor apparently requires that the coating is provided on a tungsten conductor which is free at its surface of absorbed gases and of oxides and other impurities.